Although recognized for decades that regulation of salt transport is crucial for maintenance of normal blood pressure, very little is understood about sodium transporter regulation during natriuresis or generation of hypertension. A rapid increase in blood pressure inhibits salt and fluid reabsorption in the proximal tubule (PT). This response is critical to the operation of tubuloglomerular feedback (TGF) by providing an error signal (increased delivery of NaCl) to the macula densa to sustain vasoconstriction in the afferent arteriole. The response also contributes to pressure natriuresis, which is altered in hypertension. The thick ascending limb of the loop of Hen1e (TALH) is involved in the feedback response to the extent that it does not completely reabsorb the increased load leaving the PT. The objective of this proposal is to define the cellular mechanisms responsible for the natriuresis that occurs in response to an acute increase in arterial pressure, and to determine whether the mechanisms are altered in pre- hypertensive rats. We have established that an increase in blood pressure rapidly decreases PT Na,K-ATPase activity and alpha and beta subunits' abundance in basolateral membranes and decreases apical sodium hydrogen exchangers associated with a redistribution of both sodium transporters to putative internal membranes. These results demonstrate the feasibility of dissecting the cellular mechanisms of pressure natriuresis. We will apply techniques of subcellular fractionation, immunocytochemistry, fluid phase endocytosis, and molecular biology to answer the following: Aim 1. Is the rapid decrease in PT sodium transport that accompanies an acute increase in arterial pressure due to a redistribution of apical and basolateral sodium transporters to endosomal pools, and is the response sodium transporter specific or a generalized internalization of surface membranes? Aim 2. Does the TALH respond to a pressure challenge and/or increased volume flow by regulating surface expression of sodium transporters? Aim 3. Is the response of PT and TALH to acute pressure challenge blunted in prehypertensive rats? We will study the fructose fed rats, and the Milan Hypertensive Strain rats (mutation in adducin) to test the hypothesis that altered abundance of sodium transporters, their distribution or interaction with cytoskeleton may blunt the response to an acute pressure challenge. Accomplishing the aims will fill gaps in our understanding of how altered ion transport is involved in the generation and maintenance of hypertension (salt sensitivity) and will identify components involved in the response that can be tested as "candidate genes" that may be involved in the disease process.